Electrode for peroxide generator and method for preparing it

ABSTRACT

In the disclosed electrochemical cell for the production of an alkaline solution of peroxide, especially on-site production, the electrolyte is divided into an aqueous alkaline catholyte and an aqueous alkaline anolyte, and the cathode is a gas-diffusion electrode. The active material of the electrolyte side of the gas-diffusion cathode comprises a particulate catalyst support material having a surface area of about 50 to about 2000 m 2  /g, and, deposited on the particles of this support material, 0.1 to 50 weight-%, based on the weight of the active layer, of gold or gold alloy particles having an average size &gt;40 but less than about 200 Å. These gold or gold alloy particles are substantially selectively catalytic for the reduction of oxygen to peroxide (e.g. HOO.sup.⊖). The electrolyte flow patterns are designed to avoid loss of peroxide resulting from oxidation at the anode. In the operation of the cell, a product with a hydroxyl:perhydroxyl ratio lees than 2:1 can be obtained.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

Work related to this invention was supported by a grant or award fromthe National Science Foundation (NSF Contract Nos. ISI-9060179 andISI-9203023).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of our copending application Ser. No.08/633,563, filed Apr. 17, 1996, which is in turn a continuation of ourapplication Ser. No. 08/276,178, filed Jul. 15, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrochemical cells designed to synthesizeperoxide (HOO.sup.⊖, O₂.sup.⊖, or H₂ O₂ dissolved in an alkaline medium)by cathodic reduction of an oxygen-containing gas and to processes foroperating such cells. An aspect of this invention relates toelectrochemical cells for the synthesis of peroxide wherein theelectrolyte is divided into an aqueous alkaline catholyte and an aqueousalkaline anolyte. Another aspect of this invention relates to a processfor synthesizing peroxide which can be operated at relatively low cellvoltages and relatively high current densities and efficiencies.

2. Description of the Prior Art

It has long been known that hydrogen peroxide can be synthesizedelectrochemically, taking advantage of modern advances inelectrochemical cell technology. The patent literature published on thissubject in the late 1960's and early 1970's took into consideration thepossibility of using a gas-diffusion cathode. A "gas-diffusionelectrode" is normally considered to comprise a structure which is gaspermeable on one major surface (sometimes called the "gas side") andelectrocatalytic on the opposite major surface, which opposite surfaceis in contact with the electrolyte and is sometimes called the"electrolyte side". The electrolyte is permitted to permeate into theelectrolyte side to a degree sufficient to provide a multi-phaseinterface between a gaseous reactant, a solid electrocatalytic material,and the electrolyte (which is generally a liquid). However, significantpermeation of electrolyte through pores or interstices within thecatalytic material of the gas-diffusion electrode at a significant flowrate is neither necessary nor desirable.

For a representative sampling of disclosures from this late-1960's early1970's period, see the several U.S. patents issued to Grangaard, e.g.U.S. Pat. No. 3,459,652 (Aug. 5, 1969), U.S. Pat. No. 3,462,351 (Aug.19, 1969), U.S. Pat. No. 3,507,769 (Apr. 21, 1970), U.S. Pat. No.3,592,749 (Jul. 13, 1971), and U.S. Pat. No. 3,607,687 (Sep. 21, 1971).These disclosures typically contemplate a generally free-flowingcatholyte which takes up peroxide (generally in the form of HO₂.sup.⊖dissolved in the alkaline catholyte) and is withdrawn from the cell forthe purpose of recovering a product which is intended to be directlyuseful in industry, e.g. as an alkaline bleach solution.

Experts in the electrochemical synthesis art found the performance ofthe Grangaard cells to be very disappointing, however, and by themid-1970's, even the fundamental principles upon which the Grangaardconcepts were based were being called into question. For example,according to O1oman and his coworkers, see U.S. Pat. Nos. 3,969,201 and4,118,305, issued Jul. 13, 1976 and Oct. 3, 1978, respectively, theGrangaard cells produced an aqueous alkaline product having a peroxideconcentration of only about 0.5% with an NaOH/H₂ O₂ ratio (by weightpercent) of 4:1 (cf. U.S. Pat. No. 3,459,652). As is known in the art,some uses of bleaching solution, e.g. in the pulp and paper industry,generally call for much higher concentrations of peroxide and/or forNaOH/H₂ O₂ ratios in the range of about 1:1 to about 2:1. Oloman et al,among others, questioned the basic idea of utilizing a gas-diffusioncathode of the classical structure wherein catholyte merely permeatesinto the electrode structure from the electrolyte side. Thus, by themid- to late 1970's, prior art workers were directing their attention tocathode structures constructed from a fluid-permeable, electricallyconductive mass (e.g. a bed of conductive catalytic particles or afixed, porous conductive catalytic matrix) with sufficient porosity topermit a constant trickle or flow of electrolyte through the entirevolume (or most of the volume) of the cathode mass. In anelectrochemical cell provided with such a fluid-permeable, electricallyconductive cathode mass, the cathode can, if desired, fill up the entirecathode compartment, so that all or most of the catholyte is confined tothe interior of the cathode mass.

In subsequent developments based upon the packed-bed or porous matrixconcept of a cathode, the cathode was in some cases placed in contactwith a non-conducting porous matrix (such as a felt) or was employed ina cell having in essence a single electrolyte rather than an electrolytedivided into catholyte and anolyte.

In many patent disclosures illustrating the packed-bed or porous matrixconcept of a cathode, the product (generally an alkaline solution ofperoxide, most typically catholyte which has been passed through thecathode mass) is collected from an end or edge or other portion of thecathode mass, rather than from a generally free-flowing catholyte whichhas merely contacted and/or permeated to some degree a surface of thecathode. Alternatively, the product is essentially catholyte which haswicked through a non-conductive, porous mass such as a felt which is incontact with the cathode.

Peroxide-generating cells containing packed-bed or porous-matrixcathodes have in recent years become commercially available for use ason-site peroxide generators, and considerable effort has gone into theoptimization of their performance. However, these commercially availablecells operate at overall cell potentials (E_(cell)) of about 2.0 V andcurrent densities not significantly exceeding 60 amperes per square foot(about 64.5 mA/cm² =645 A/m²). Even assuming a current efficiency of 85to 90%, a large amount of electrode surface area is required in atypical commercial installation, resulting in higher capital costs tothe industrial user.

Moreover, the packed-bed or porous matrix concept of cathodeconstruction has not provided any improvement in quality control ascompared to cells utilizing gas-diffusion cathodes. The packed bed orporous matrix can develop "hot spots" in which current densities, etc.are higher than average for the bed or matrix as a whole, therebycreating the risk that some part of the cathode might become "starved"for three-phase interface sites and can place the entire bed or matrixat risk of catastrophic failure. This risk can be reduced through theuse of a significant stoichiometric excess of oxygen, but non-uniformconsumption of oxygen throughout the cathode bed or matrix stillcontributes to poor quality control. In addition, bipolar cellconstruction, with stacking of cells for more efficient overalloperation, is problematic, due to the variability in the performance ofindividual cells.

Accordingly, despite significant advances in the field of on-siteelectrosynthesis of peroxide over the last twenty years, cellperformance is still in need of substantial improvement.

SUMMARY OF THE INVENTION

It has now been discovered that the performance of electrochemicalperoxide synthesis cells and processes in terms of operating potentials,range of current densities, cathode lifetime, and the resulting systemcapital costs can be markedly improved through the use of anelectrochemical catalytic material containing gold particles and throughthe use of a generally free-flowing catholyte which inhibits loss ofperoxide due to oxidation of perhydroxyl ion at the anode of the cell.Owing to a unique cathode preparation process incorporating the goldcatalyst, the relatively poor performance of the Grangaard cells is notobserved during the practice of this invention.

Thus, an electrochemical cell of this invention comprises:

a partitioning means (such as a fluid-permeable separator) forpartitioning the cell into an anode compartment and a cathodecompartment, the anode compartment containing an anode and an aqueousalkaline anolyte,

a cathode compartment defining a space containing a generallyfree-flowing aqueous alkaline catholyte, a gas-diffusion cathode havingtwo major surfaces (the cathode occupies, at most, only a minorproportion of the volume of this space), the space defined by thecathode being constructed and arranged to permit generally unrestrictedflow of the aqueous alkaline catholyte across one of the major surfacesof the gas-diffusion cathode, i.e. the "electrolyte side" (the othermajor surface of the cathode, i.e. the "gas side", is in contact withoxygen-containing gas). A catholyte withdrawal means permits withdrawalof the spent catholyte, which is the desired product of peroxidedissolved in an aqueous alkaline medium.

The electrolyte side of the cathode comprises an electrochemicallyactive material which is composed of:

a particulate catalyst support material having a surface area, by theB.E.T. method, of about 50 to about 2000 m² /g,

deposited on the particles of catalyst support material of the activelayer, 0.1 to 50 weight-%, based on the weight of the active layer, of aparticulate elemental metal comprising gold particles having an averagesize, measured by transmission electron microscopy, which is greaterthan about 4 but less than about 20 nanometers, this particulateelemental metal being substantially selectively catalytic for thereduction of oxygen to peroxide ion or hydrogen peroxide, and

preferably, a gas-permeable, electrically conductive support material onwhich the active material is deposited. The active material ispreferably rendered hydrophobic by including in it at least 30% byweight, based on the weight of the active layer, of hydrophobic polymer.

Electrical leads in electrical contact with the anode and the cathode ofthe cell are provided for the external electrical circuit. If desired,the cell can be bipolar and can be electrically connected to one or moreadditional cells.

In the process of operating this electrochemical cell, the overall cellpotential (E_(cell)) need not exceed 2 volts and can be less than 1.5 V;current densities can range from 700 to 2,000 A/m² (70 to 200 mA/cm²) ormore, yet current efficiencies do not suffer and are typically in therange of about 85 to about 95%. In addition, current densities in excessof 300 mA/cm² are attainable. Since the size, and, therefore, the cost,of a commercial system is a direct linear function of the electrodearea, a substantial improvement in sustainable current density willsignificantly reduce the system capital costs. This outstandingperformance can be obtained under near ambient operating conditions,including temperatures in the range 35-40° C. The hydroxide/peroxideratio of the resulting product is well-controlled but variable inaccordance with the desired use and, hence, can vary from about 1.6:1 toall higher values. The peroxide yields associated with these productratios are typically 3-5 wt %.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying Drawing, wherein like reference numerals denote likeparts in the various embodiments of the invention,

FIG. 1 is a schematic view of a relatively simple embodiment of anelectrosynthesis cell of this invention which employs ananolyte-to-catholyte flow and therefore requires only one electrolyteinlet and one electrolyte outlet,

FIG. 2 is a schematic view, similar to FIG. 1, of an especiallyhigh-performance embodiment of an electrosynthesis cell of thisinvention which employs anolyte and catholyte flows which are generallyseparate but are in fluid communication with each other, and

FIG. 3 is a greatly enlarged fragmental cross-sectional view of thecathode 30 shown in FIG. 1.

DETAILED DESCRIPTION

Turning first to the Drawing, FIG. 1 illustrates an embodiment of anelectrochemical synthesis cell 10 of this invention which is desirablefrom the standpoint of simplicity. Cell 10 comprises an anode 11, agas-diffusion cathode 30, a fluid-permeable cell divider or separator15, an anode lead 39, a cathode lead 37, an inlet 21 for the flowingelectrolyte, an outlet 23 located near anode 11 for the release ofoxygen generated during cell operation, an outlet 25 for the flowingelectrolyte, an inlet 27 located near the "gas side" of cathode 30 forthe introduction of an oxygen-containing gas, and an outlet 29 forexcess oxygen-containing gas. The electrolyte introduced in to inlet 21is supplied from a source 12 external to cell 10.

Separator 15 divides cell 10 into an anode compartment 17, which isconstantly filled with continuously flowing anolyte, and cathodecompartment 19, which is constantly filled with continuously flowingcatholyte, but separator 15 is porous and has sufficient porosity topermit flow-through of electrolyte and, hence, migration of ions betweencompartments 17 and 19. Cathode 30 (see also FIG. 3) has a typicalgas-diffusion electrode structure comprising an electrically conductivesheet-like gas-permeable support material 31 upon which is superposed anelectrocatalytically active material 33 comprising high surface areaparticles upon which tiny particles of gold or a gold alloy have beendeposited. Active material 33 preferably also contains a hydrophobicbinder material, e.g. a highly fluorinated olefinic polymer or otherpolyhalohydrocarbon such as polytetrafluoroethylene, in an amountgreater than 25% by weight, based on the weight of the active material33, most preferably about 50 to 70 weight-%. Amounts greater than about75 or 80 weight-% can have an unacceptable adverse impact uponperformance without increasing cathode life significantly as compared tothe 70 weight-% level of binder. Support material 31 can be a carboncloth, carbon paper, or teflonated metal screen which serves as acurrent collector and which is sufficiently hydrophobic (or has beentreated with a hydrophobic polymer such as a polyhalohydrocarbon e.g.polytetrafluorethylene) to prevent flow-through of catholyte. Cathodelead 37 is electrically connected to the support material 31.

An external circuit means (not shown) provides an electrical pathwaybetween anode 11 and cathode 30.

In operation, a fresh aqueous alkaline medium is introduced throughinlet 21 into anode chamber 17 to refresh the anolyte, which isconstantly being depleted of hydroxyl ion in accordance with half-cellreaction (1):

    2OH.sup.⊖ →1/2O.sub.2 +H.sub.2 O+2e.sup.⊖(1)

and which does not receive an adequate flow of hydroxyl ion from thecathode compartment 19, even under the most ideal circumstances, sincethe half-cell reaction occurring at cathode, for every two electronsaccepted, produces only one mole of hydroxyl ion in accordance withhalf-cell reaction (2):

    O.sub.2 +H.sub.2 O+2e.sup.⊖ →HO.sub.2.sup.⊖ +OH.sup.⊖.                                        (2)

The electrolyte introduced into anode compartment 17 becomes part of theanolyte, but it also passes through the pores of separator 15 intocathode compartment 19 and out of the cell 10 through outlet 25 at arate selected to limit the limit migration of perhydroxyl ion into theanode component 17. That is, the direction of flow of electrolyte ismaintained (with the aid of a pump) in anolyte-to-catholyte direction,which is counter to the natural diffusion of hydroxyl ions from cathodecompartment 19 to anode compartment 17 and, more importantly, is counterto the tendency of perhydroxyl ions (H0₂.sup.⊖) to migrate into theanode chamber 17, where they are exposed to possible oxidation tooxygen. This undesirable side reaction, represented below by half-cellreaction (3)

    HO.sub.2.sup.⊖ +OH.sup.⊖ →O.sub.2 +H.sub.2 O+2e.sup.⊖                                        (3)

results in the loss of valuable peroxide product and is highlydetrimental to the objectives of this invention.

The spent catholyte, so to speak, which exits the cell through outlet25, is the desired product of the electrosynthesis. Thus, outlet 25serves as the means for recovering the alkaline solution of peroxidewhich can be used as a bleaching agent or oxidizing agent orsolubilizing agent for treating pulp, paper, and other industrialproducts. The alkalinity and the peroxide content in outlet 25 can becontrolled, according to this invention, over a surprisingly broad rangeby controlling the parameters of cell operation, including electrolyteflow and the like. Generally speaking, the solution in outlet 25 has analkalinity and a peroxide content which has been carefully matched tothe industrial needs prevailing at the site of cell 10, so that cell 10can serve as an on-site peroxide generator, making just enough peroxideto satisfy current demand, no more and no less. On-site generation ofperoxide avoids storage of peroxide and purchase of highly concentratedperoxide from outside sources, both of which are undesirable and caneven be hazardous.

The catholyte in cathode chamber 19 flows along the surface of activematerial 33 of cathode 30 and permeates into active material 33 to aconsiderable degree, but does not flow through cathode 30 in the mannercatholytes fed to packed-bed or porous matrix cathodes, for severalreasons. First, support material 31, though permeable to gas, ishydrophobic and will not permit aqueous media to pass through it.Second, the volume of active material 31 is very small compared to thevolume of a packed-bed cathode, and it will not accommodate a voluminousflow of liquid. Moreover, the porosity of cathode 30 is generally in theform of very fine pores which are better suited to capillary action thanhigh flow rates. In any event, cathode chamber 19 and cathode 30 aredesigned to provide a fairly rapid flow of catholyte parallel to thesurface of cathode 30.

The oxygen-containing gas introduced through inlet 27 contacts thegas-permeable support material 31 of cathode 30 and permeates into theactive material 33. Because of the permeation of oathclyde into activematerial 33, this portion of cathode 30 provides a multitude of sitesfor a three-phase interface of catholyte, oxygen-containing gas, andsolid catalytic material. The oxygen is reduced to peroxide at thisthree-phase interface, and the perhydroxyl ions diffuse into thecatholyte. Turning now to FIG. 2, the cell 50 shown in this Figure ispreferred for more controllable contact times between catholyte andcathode 30, hence a more controllable production of peroxide. Inaddition, the cell design shown in FIG. 2 may allow for the decouplingof current efficiency and product ratio due to the independent catholyteand anolyte flows. The flow of catholyte through cathode compartment 19is rapid enough to prevent any significant migration of perhydroxyl ioninto anode chamber 17, thereby eliminating the need for a countercurrentflow of electrolyte. In this embodiment, fresh anolyte (from source 14)of independently selected alkalinity continuously enters through inlet22, thereby keeping the hydroxide ion concentration in anode compartment17 from being excessively depleted, and the spent anolyte flows outthrough outlet 24. On the cathode side of cell 50, fresh catholyte (fromsource 16), also of independently selected alkalinity, enters throughinlet 26 and the product of the electrosynthesis flows out throughoutlet 28. The fresh influx of catholyte through inlet 26 and theconstant efflux of catholyte prevent excessive buildup of hydroxide ionin cathode compartment 19, which is very important with respect tomaintaining proper control over the hydroxyl/perhydroxyl ratio of theproduct effluent. As indicated previously, this ratio can be varied fromas low as 1.6:1 to all higher values. Hydroxyl/perhydroxyl ratios ashigh as 2:1 or even 1.8:1 are unsuitable for many important industrialuses of alkaline peroxide solution, whereas hydroxyl/perhydroxyl ratiosas low as 1:1 can create problems in the operation of cells 10 or 50.Accordingly, the particularly preferred hydroxyl/perhydroxyl ratio is inthe range of 1.2:1 to 1.7:1.

In this invention, it is preferred to utilize the essentially pureoxygen produced at anode 11. This objective is most easily accomplishedby circulating the oxygen through a conduit system 41, external to cell50, to enrich the oxygen-containing gas introduced through inlet 27 onthe cathode side of cell 50. System 41 can be used in cell 10 also, butfor simplicity of illustration, system 41 is shown only in associationwith cell 50.

Except for the flowing electrolyte arrangement (compare inlet 21 andoutlet 25 of cell 10 with inlets 22 and 26 and outlets 26 and 28 of cell50), it will be noted that cells 10 and 50 can otherwise besubstantially identical in structure and operation.

The loading of gold or gold alloy in the active material 33 of cathode30 (of FIGS. 1 or 2) can range from 0.1 to 50% by weight, based on theweight of active material 33. Loadings in the range of 2 to 20% arepreferred. Details of the structure of cathode 30 can best be seen inFIG. 3, which shows hydrophoblc support material 31 (optionally treatedwith a hydrophobic polymer such as polytetrafluoroethylene or a similarfluorinated hydrocarbon), active material 33, and current collector orlead 37. The details shown in FIG. 3 relate to cathode 30 of both FIGS.1 and 2, because the structure of cathode 30 is identical in both ofthose Figures.

Preferred materials for the separator 15 of FIG. 1 includealkali-resistant porous inorganic oxides and silicates and the like andporous organic polymeric materials which resist strong alkalis, e.g.microporous polyolefins. The separator 23 of FIG. 2 is a like material,but cation exchange membranes can also be used in the embodiment of FIG.2. The preferred anode 11 is an alkali-resistant bulk metal such asnickel or a noble metal, which is preferably porous (e.g. a metal screenor mesh or "expanded metal"). The preferred electrolyte is an aqueousalkaline medium such as an aqueous solution of an alkali metalhydroxide, a highly water-soluble alkaline earth metal hydroxide, or ahighly water-soluble quaternary ammonium hydroxide. Alkali metalhydroxide solutions are preferred, and sodium particularly preferredfrom the cost standpoint.

The preferred oxygen-containing gas introduced through inlet 27 issubstantially pure oxygen or a mixture of oxygen with an essentiallyinert gas such as nitrogen or argon. A particularly convenient way toobtain a suitable O₂ /N₂ mixture is to remove the carbon dioxide contentof air, e.g. through a compression/condensation or alkaline-scrubtechnique. since carbon dioxide can form carbonates in an alkalineelectrolyte, and since some carbonates (even alkali metal carbonates)can be less soluble than the corresponding alkali metal hydroxides atcell operating temperatures, resulting in precipitation of carbonatesalt in the pores of the cathode, the presence of carbon dioxide in theelectrolyte is preferably avoided.

The Active Material

It is well known in the art that a gold-containing electrocatalyst of agas-diffusion electrode in an alkaline electrolyte can facilitate theselective reduction of oxygen to peroxide (O₂ ⁶³, HO₂ ⁶³, and/or H₂ O₂,are all referred to in this application as "peroxide"), e.g. inaccordance with half-cell reaction (2), above.

This reaction involves a "two electron change"--as opposed to the "fourelectron change" of the complete reduction of oxygen to hydroxide orwater. It is also known that gold crystals can catalyze thefour-electron change at the (100) face of these crystals, whereas theother crystalline faces (and polycrystalline gold) are specific for thetwo-electron change. See U.S. Pat. No. 5,041,195 (Taylor et al), issuedAug. 20, 1991, the disclosure of which is incorporated herein byreference. Although gold-containing electrocatalysts could theoreticallybe very effective in improving the performance of a peroxideelectrosynthesis cell, and although gold is very stable (resistant tocorrosion) in alkaline electrolytes, there appears to be very littlediscussion in the patent literature regarding the use of suchelectrocatalysts for this purpose, particulate carbon being the materialmost typically mentioned as suitable for catalysis of theelectrosynthesis. Despite its promise of improved peroxide production,formulation of gold-containing electrocatalytically active materials ofsuitable efficiency can be problematic, and prior art appears to providevery little detailed guidance in this regard.

Surprisingly, the basic principles involved in supporting tiny goldparticles on high surface-area carbon, disclosed in the 5,041,195 patentcited above, have been found to be highly useful in the context of thisinvention, even though these principles relate to gold catalystsspecific for the four-electron reaction (complete reduction to hydroxideor water) rather than the two-electron reaction (partial reduction toperoxide). It has been found that relatively minor modifications of thetechniques described in the 5,041,195 patent can provide gold or goldalloy particles which selectively catalyze the two-electron,peroxide-forming reaction.

The techniques of the 5,041,195 patent are directed toward maximizingthe formation of tiny monocrystals (averaging less than 50 Å, moretypically <40 Å, in size) which are selective for the four-electronreaction. To obtain supported gold or gold alloy particles which arespecific for the two-electron change but are otherwise prepared inaccordance with the 5,041,195 patent, one can utilize graphitized carbonas the support material and/or select conditions favoring the formationof somewhat larger metallic particles (i.e. particles having an averagesize of at least 40 Å, but generally less than 200 Å and preferablyabout 50 to about 150 Å). It has been found that the tinymonocrystalline particles produced according to the 5,041,195 patent canserve as nucleation sites for the "growth" of somewhat larger particlesof almost any desired size within the aforementioned preferred range of50 to 150 Å. For example, the technique described in the 5,041,195patent can be followed exactly, and the resulting gold-containingnucleation sites can be subjected to a heating step which sinterstogether some of tiny metallic particles, thereby increasing theiraverage particle size to >50 Å (>5 nm).

Suitable catalyst support materials include high surface area carbon andother finely divided inorganic materials (e.g. metallic oxides or thelike). Finely divided carbon is presently preferred due to itscommercial availability in a range of particle sizes and due to itsdesirable inherent catalytic properties. When measured by the B.E.T.method, carbon powders such as furnace blacks, lamp blacks, acetyleneblacks, channel blacks, and thermal blacks can provide surface areasranging from 50 m² /g up to almost 2000 m² /g, surface areas >200 m² /g,e.g. >600 m² /g, being preferred. The particle sizes of the carbon inthese powders can range from about 5 to about 1000 nanometers (50 to10,000 Å) but are preferably smaller than 300 nanometers in size. Sincethe surface area of the gold or gold alloy particles is normally lessthan that of the high surface area carbon, at least some carbon isexposed to the alkaline electrolyte and is subject to chemical attack,but adequate stability in alkaline media can be obtained with cathodesprepared according to this invention.

Commercially available carbon materials include BLACK PEARLS (tradedesignation), KETJENBLACK (trade designation), VULCAN (tradedesignation), "CSX", and Lurgi blacks, BLACK PEARLS and KETJENBLACKbeing preferred. These materials are described in detail in U.S. Pat.No. 5,041,195, and this description is specifically included in thesubject matter incorporated by reference from the 5,041,195 patent.

As indicated above, the preferred method for depositing (forming insitu) gold or gold alloy particles on high surface area carbon is amodification of the methods disclosed in U.S. Pat. No. 5,041,195.Generally speaking, a reducible gold compound in solution is impregnatedinto the support material with the aid of a polar solvent (e.g. analcohol or alcohol/water mixture) having adequate wettingcharacteristics with respect to the support material; the solvent isgently evaporated at moderate temperatures (higher temperatures can beused to obtain somewhat larger gold particles); and the resulting dry orsubstantially dry material is subjected to chemical reduction with areducing gas such as substantially dry hydrogen. The reducible goldcompound can be chlorauric acid (HAuCl₄), a salt of this acid, a goldhalide, or the like. The resulting gold particles can be smaller than 40Å in size, but, as indicated above, they can also serve as nucleationsites for further particle growth (e.g. by sintering, as indicatedpreviously). The most preferred active material thus comprises a ratherhigh-surface area gold deposited on an even higher surface area carbon.

As indicated previously, the active material preferably contains atleast 30%, generally about 50 to 70 weight-% of polymeric hydrophobicbinder, The binder can be introduced into the active material, duringits preparation, as a suspension of fine particles of hydrophobicpolymer in a carrier such as water or an organic solvent.

The peroxide yield is variable and can readily provide the range of 3-5wt % and NaOH/H₂ O₂ ratio not exceeding about 2:1 typically desired bythe pulp and paper industry.

The following Example illustrates the principle and practice of thisinvention without in any way limiting its scope.

Part A--Cathode Preparation Step

Cathodes were prepared as gas-diffusion electrodes (GDE's) fromcarbon-fiber paper as the gas-permeable support layer and a singledeposit of an electrocatalyst layer (electrochemically active material).The preparation process was begun by first sieving the particulatecarbon, which is itself an but was used as catalyst support material inthis Example, through a -170 mesh (U.S. or Tyler) screen. Theparticulate carbon support material was then dispersed in an acidifiedaqueous solution (65 millimolar sulfuric acid). Ultrapure water was usedto make this acidified aqueous solution. The carbon was added to theacidified aqueous solution with stirring and ultrasonification such thatwhen the entire solution was applied to the carbon-fiber paper theelectrocatalyst loading was 5 mg/cm². For each 10 cm×10 cm of electrodearea the carbon mass was typically 0.517 grams. Apolytetrafluoroethylene (PTFE) binder was then added (30, 50, or 70 wt%), using a dilute aqueous suspension of "TFE-30" (trade designation ofthe DuPont Company), and the suspension was mixed with stirring andultrasonification. The resulting blend was filtered and the filtrate(deposit on the filter paper) was transferred onto wet-proofed, porouscarbon-fiber paper substrate (Toray Industries) to form a uniform layer.The electrode was subsequently cold pressed at 1200 pounds pressure foreach 16 in² of electrode area (75 psi, 517 kPa) until dry, hot pressedup to 1200 pounds pressure (75 psi, 517 kPa) for five minutes at 100 C.,and sintered stepwise at 100° C. for one hour, 200° C. for an hour, thenfinally at 300° C. for fifteen minutes.

To produce an electrocatalyst layer with small gold particles requiredthe previously-described modification to the process described in U.S.Pat. No. 5,041,195. This patent details the techniques directed towardmaximizing the formation of tiny monocrystals (averaging less than 50 Å,more typically <40 Å, in size) which are selective for the four-electronreaction. To obtain supported gold or gold alloy particles which arespecific for the two-electron change but are otherwise prepared inaccordance with the U.S. Pat. No. 5,041,195, one can utilize graphitizedcarbon as the support material and/or select conditions favoring theformation of somewhat larger metallic particles (i.e. particles havingan average size of at least 40 Å, but generally less than 200 Å andpreferably about 50 to about 150 Å). It has been found that the tinymonocrystalline particles produced according to the U.S. Pat. No.5,041,195 can serve as nucleation sites for the "growth" of somewhatlarger particles of almost any desired size within the aforementionedpreferred range of 50 to 150 Å. In this Example, the technique describedin the U.S. Pat. No. 5,041,195 patent can be followed exactly and isincorporated herein by reference, except that the resultinggold-containing nucleation sites were subjected to a heating step in aninert atmosphere at temperatures ranging from 300° to 1200° C. whichsintered together some of tiny metallic particles, thereby increasingtheir average particle size to >50 Å (>5 nm) .

Part B--Peroxide Generator Apparatus

Two size systems were used to test and evaluate the cathodes and theirperformance. Both types employed three-compartment, flowing electrolytedesigns as shown in FIG. 2 of the Drawing. The system of smaller sizewas constructed by the present applicants and was used for smallcathodes of area not exceeding 3.0 cm². These cells were constructedfrom polymethylmethacrylate (LUCITE®) with a total cell volume ofapproximately 10 ml. A constant electrolyte flow was maintained by aperistaltic pump and the electrolyte was recirculated from acontinuously mixed 600 ml reservoir. As shown in FIG. 2, the electrolytecompartment was partitioned into two separate compartments by a cationexchange membrane, NAFION® 117 (trademark of the DuPont Company forfluorinated cation exchange polymer having pendent groups containing--SO₃ H radicals). The anode electrode was a solid nickel sheet. Theelectrolyte temperature was maintained above 40° C. byfeedback-controlled, in-line heaters through which the electrolytepasses. Electrolyte temperature was monitored by insertion ofthermocouples into the fluid entrances and exits. A mercury/mercuryoxide electrode was used as reference. The internal resistance of thecells was monitored through the use of an auxiliary platinum wireelectrode in combination with an 800 IR Measurement System(Electrosynthesis Corp.). The cells were operared in constant currentmode with a 3-Amp power supply. Four such cells were constructed and,typically, operated simultaneously with a computer-based dataacquisition system.

A larger cell system with cathode area up to 100 cm² was prepared byadaptation of a commercial electrolyzer. This system was referred to asa process development unit (PDU) and comprised an EA ElectroCell MP (EACorp., Sweden, obtained through the Electrosynthesis Corp.), suitablymodified as described below. This cell was chosen over other possiblecommercial alternatives as it is more readily adapted for gas diffusionelectrodes. The principal difference between this and the smallapparatus was the variable cathode-anode spacing. The power supply wasproportionately larger and was purchased from Power Ten Inc. In additionto a cation exchange membrane, NAFION® 117 (DuPont, see previousdescription), an alternative, relatively inexpensive separator was alsosuccessfully used in the PDU. This was 7-mil (179-μm) thick TESLIN® (PPGIndustries Inc.) which is a silica-based, porous, polymeric material.The TESLIN® separator was determined to perform as well as the NAFION®117 membrane. Several changes to the EA ElectroCell MP (commercialsystem) were necessary to fully adapt it to serve ae the PDU (unit forcaustic peroxide generation via GDE's). These changes were:

(1) The picture frame cathode assembly was replaced with a machinedgraphite block. The addition of the graphite block provides physicalsupport of the carbon backing layer necessary to compensate forhydrostatic pressure. The block also ensures a leak-free seal betweenthe catholyte and gas compartments. We chose graphite to provideelectrical contact and for corrosion resistance. The block was drilledwith a matrix of holes to allow gas to uniformly contact the rearsurface of the GDE. This matrix of holes accounted for approximately 10%of the active cathode area. Therefore, the cathode-side of the block wasmachined to form channels leaving only small pegs in which to contactthe GDE. Total surface area of these pegs was approximately 10%. Thechanneling provided uniform gas-flow across the back side of the GDE and90% cathode area utilization.

(2) The separator frames were replaced with components more compatiblewith elevated temperature operation. These frames define thecathode-anode gap and have mesh inserts which promote turbulence. Thereplacement frames were equipped with reinforced flow canals to resistdeformation under pressure and elevated temperature.

(3) The solid nickel sheet anode was drilled with a matrix of holes toresemble a mesh. This modification was necessary to allow evolved gas toescape behind the anode, away from the membrane.

(4) Accurate temperature monitoring was also required. This wasaccomplished by insertion of Teflon®-tipped thermocouple probes directlyinto the caustic entrance and exit ports of the PDU.

(5) Shut-down of the PDU was necessary for changing gas cylinders andelectrolyte and for short-term unattended operation, such ae cathodebreak-in. At the open circuit potential, the cathode begins to oxidizeimmediately. Removing the electrolyte was insufficient to prevent this,since the cathode and membrane retain moisture for long periods. The PDUstand was modified to allow draining and flushing of the cell withdeionized water. A four-way valve was also added to the gas feed systemto allow the cell to be nitrogen-purged to displace oxygen.

Part C--Performance Data

Cathodes composed of up to 10 wt % gold on KetjenBlack (tradedesignation for particulate carbon) were prepared as described in Part Aand were mounted in the PDU apparatus as described in Part B. The systemwas operated at cell temperatures of 45°-50° C., with a total anode tocathode gap of 0.8 cm, and for an input electrolyte concentration of 10wt % NaOH. Cell polarizations (i.e. total cell potential versus currentdensity) of the gold on carbon electrodes were equal to or improved incomparison to the carbon black alone. In addition, the gold-catalyzedelectrodes were capable of sustained operation at current densitiesequal to or exceeding 300 mA/cm². The performance of a 10 wt %gold-on-carbon electrode was 1.34 V at 100 mA/cm², 1.73 V at 200 mA/cm²,and 2.08 V at 300 mA/cm².

The reporting operating conditions of the trickle bed cathode system are2 V at 60 mA/cm². The operating potential for the gold-on-carbonelectrode tested here is approximately 1.06 V at 60 mA/cm². Thus, forthe same current density a factor of two in power savings per part byweight of peroxide product can be realized. Alternatively, highercurrent operation may be performed to reduce overall system size therebyreducing the generator capital costs. The 300 mA/cm² current densitydata indicates that in comparison to the trickle bed system, a GDE-basedgenerator system may be reduced in total electrode area by up to afactor of five. This will translate to a significant reduction ingenerator system capital costs since these costs are a linear functionof electrode area. Further performance improvements, principally due todecreases in the IR losses, are expected as the gap is decreased to areasonable commercial limit of 0.5 cm.

Cathode lifetime and cathode cost are critical parameters in determiningcommercial system operating costs. The more frequently the cells have tobe replaced and the more expensive the cells cost, the higher the costof the electrogenerated peroxide will be and the poorer the comparisonwill be with simple purchase and storage. Extensive testing of theelectrodes in both size systems described in Part B have identifiedtotal electrode hydrophobicity to be a critical factor in determiningcathode lifetime. The most successful cathodes have incorporatedhydrophobic polyhalohydrocarbon polymer, preferably PTFE, into thecarbon paper and contain >30 wt.-% PTFE (e.g. 50 and 70 wt.-%) in theelectrocatalyst layer, i.e. in the electrochemically active material.With active material containing 50 to 70 wt.-% PTFE, the cathodes willsurvive for several thousand hours with only minor decreases inperformance characteristics.

In view of the relatively low cost of the catalyst components describedabove, the use of gold in the active material does not increase themanufacturing cost of the gas-diffusion cathode beyond present goals foron-site peroxide generator markets in the U.S. and elsewhere.

What is claimed is:
 1. A process for preparing a gas-diffusion cathodefor the reduction of an oxygen-containing gas to peroxide,comprising:impregnating into a catalyst support material having asurface area, by the B.E.T. method, of about 50 to about 2,000 m² /g asolution containing a reducible gold-containing compound and a polarsolvent and depositing said gold-containing compound, in solution, onsaid catalyst support material, evaporating said solution and obtaininggenerally dry deposits of said gold-containing compound on said catalystsupport material, reducing said gold-containing compound thus depositedon said catalyst support material with a reducing gas until metallicgold-containing particles in the size range of up to 40 Å are obtained,and combining said gold-containing particles to increase their size toan average size, measured by transition electron microscopy, which isgreater than 50 but less than about 200 Å.
 2. A process according toclaim 1, wherein the gold-containing particles resulting from saidcombining step are poly-crystal particles.
 3. A process according toclaim 2, wherein said average size ranges from 50 to 150 Å.
 4. A processaccording to claim 1, wherein said combining step increases the size ofsaid gold-containing particles by sintering said particles together. 5.An electrode prepared by the process of claim
 1. 6. An electrodeaccording to claim 5, wherein the gold-containing particles which havebeen increased in size are poly-crystal particles.
 7. An electrodeaccording to claim 6, wherein said average size ranges from 50 to 150 Å.8. An electrode according to claim 6, wherein said gold-containingparticles which have been increased in size are sintered particles. 9.An electrode according to claim 5, wherein the amount of saidgold-containing particles which have been increased in size ranges from0.1 to 50 weight-%, based on the combined weight of the gold-containingparticles and the catalyst support material.
 10. An electrode accordingto claim 9, wherein said amount ranges from 0.1 to 10 weight-%.
 11. Anelectrode according to claim 9, wherein the amount deposited on theparticles of catalyst support material of said active layer ranges from0.1 to 10 weight-%.
 12. A gas-diffusion electrode having a first,electrolyte-compatible major surface and a second, gas-permeable majorsurface, said first surface comprising an electrochemically activematerial, said active material comprising:a particulate catalyst supportmaterial having a surface area, by the B.E.T. method, of about 50 toabout 2,000 m² /g, deposited on the particles of catalyst supportmaterial of said active layer, 0.1 to 50 weight-%, based on the weightof the active layer, of a sintered, particulate elemental metalcomprising gold, the sintered particles of sintered, particulateelemental metal having an average size, measured by transmissionelectron microscopy, which is greater than about 50 but less than about150 Å nanometers; said sintered particles having been formed bysintering together particulate elemental metal particles comprising goldand having an average particle size, prior to sintering, of less than 50Å; said sintered particles being substantially selectively catalytic forthe reduction of oxygen to peroxide ion or hydrogen peroxide.